Appropriate control over the type-II band alignment between InAs and GaSb is important for a number of applications, including the further development of midinfrared (IR) semiconductor lasers and long-wavelength photodetectors. Accurate tailoring of interface structure and composition in such heterostructures grown by molecular beam epitaxy is nevertheless problematic for several reasons. Special challenges are posed by antimony segregation at the arsenide-on-antimonide interface, indium segregation at the antimonide-on-arsenide interface, and by the desire to selectively control the purity and types of interface bonds at either heterojunction. Here, we briefly review how cross-sectional scanning tunneling microscopy may be used to examine antimony and indium segregation with atomic-scale precision in type-II quantum wells, and then explain how such measurements suggest a unique structural interpretation for the residual strain exhibited by typical mid-IR semiconductor laser active regions in x-ray diffraction. (C) 2004 American Vacuum Society.